[llvm] Add new llvm.cbrt intrinsic (PR #178396)

[David Sherwood via llvm-commits]

david-arm wrote:

 
> Having the builtin or not doesn't change anything. The intrinsic needs to have a lowering path that always works. If you can use a libcall, you could have started with the libcall in the first place. The intrinsic is adding a new burden on the backend

OK, thanks for explaining! I'll abandon this PR in favour of a different approach that avoids adding a new intrinsic. I think I can instead add an IR-level simplification that replaces calls to the llvm.pow.f32 intrinsic with a call to `cbrtf`, etc.

https://github.com/llvm/llvm-project/pull/178396